As discussed in patent application Ser. No. 12/436,375 incorporated herein by reference, the military, police and some commercial installations have vehicles that are provided with a virtual forest of antennas to cover various frequency bands. As a result there is a requirement for continuous coverage in a single antenna that operates between from the VHF bands at 30 megahertz all the way up to the 6 UHF gigahertz frequencies.
In order to be able to provide multi-band coverage, up to 4 or 5 antennas are separately utilized on a vehicle. The bands of interest for the military are the 30-88 megahertz band, the 108-156 megahertz band, the 225-450 megahertz band, the 1350-1550 megahertz band and the 1650-1850 megahertz band.
As mentioned above, there is a necessity for military, law enforcement and even commercial vehicles to be equipped with communication devices to permit operators to exchange information with a variety of different information services, command and control and dispatch centers. Also, GPS coverage is often required for geolocation. While these vehicles can employ multiple separate antennas designed to communicate effectively at a particular frequency range, there is a requirement for a single antenna that may be mounted to existing vehicles so that one antenna can have the gain of legacy antennas, while supplanting the forest of antennas previously utilized.
More particularly, a so-called Sinegars antenna typically operates between 30 megahertz and 88 megahertz, where the 30 megahertz legacy antenna has a −3 to −6 db gain over a ¼ wave monopole. The 30 megahertz legacy antenna is typically a monopole antenna whose gain is directly proportional to antenna volume. It is noted that for 30 megahertz, a quarter wavelength is 8 feet which makes a quarter wave antenna unusable in a wide variety of applications.
What is therefore required in addition to multi-band operation is an antenna whose overall height is no more than 4 or 5 feet.
It would therefore be desirable for instance to be able to replace the army AS3900A whip antenna with a single relatively short multi-band whip antenna that could provide the requisite gain.
One antenna capable of multi-band use is described in U.S. patent application Ser. No. 11/641,041 assigned to the assignee hereof. This antenna is designed to operate in the 30 to 88 megahertz band. However it is over 105 inches tall. Another problem with this antenna is that it is fabricated utilizing a number of sections of tubing that are screwed together. It has been found that these antennas are not readily fabricatable and deployable in the field due to the variability when screwing the sections together and due to the fact that from a storage point of view a 105 inch antenna is not practical.
Thus, especially for the Sincgars radio band, providing such an antenna, primarily for voice communications, has its problems. Moreover, when considering vehicle mounted antennas operating above a ground plane, variability in the ground plane configuration causes matching and radiation pattern problems because vehicle configurations can vary significantly. It therefore becomes critical as where on the vehicle to mount the antenna.
While it might be thought that any antenna could be tuned for each vehicle, such antennas are not practical and the simple solution is to simply avoid frequencies where VSWR is high, with the obvious coverage disadvantages.
Moreover, aside from its length and multi-part construction, it was found that standard meanderlines used to separate out the bands did not adequately act as traps. Thus, while various dipoles were designed to operate in various bands, the traps did not function properly to switch from a short to a trap at the band demarcations.
Secondly, especially in the middle and upper bands, the prior antenna did not exhibit sufficient gain so that the antenna could not match or exceed legacy antennas.
Further, it was found that in shortening the prior antenna, linearly downsizing the meanderlines did not result in the either sufficient gain or sufficiently low VSWR.
Moreover, it was almost impossible to tune the meanderlines once in place. The result was that pre-tuned antennas would not exhibit the required tuning when vehicle mounted.
Finally, the antenna could not pass the so-called oak-beam test, in which the antenna is to withstand repeated impact with an oak beam at 30 mph.
For these reasons the antenna design described in the aforementioned patent applications had to be abandoned and a new antenna had to be designed that would solve the problems noted above.
Note, it has been proposed to miniaturize antennas by using so-called meanderline loaded antennas exemplified by U.S. Pat. Nos. 5,790,080; 6,323,814; 6,373,440; 6,373,446; 6,480,158; 6,492,953; and 6,404,391, all assigned to the assignee hereof and incorporated herein by reference. While these meanderlines have been utilized in the past for impendence matching and tuning purposes, they were not utilized to provide chokes or traps between various dipole segments so as to make a single whip operate in a multi-band mode.
By way of background, in order to solve the multiple antenna problems noted above, and as discussed in the aforementioned patent application, a series of dipoles are mounted one on top of the other in which the antenna consists of a number of coaxially-located tubular sections, with gaps in the tubing either providing feed points for the associated dipole or for the interposition of shielded meanderlines that properly perform as chokes or traps. As a result, as one moves up in frequency, one coverts the antenna structure from a 30-190 hertz dipole to a 225-240 megahertz dipole and then to a 700 megahertz to 2 gigahertz dipole, with the chokes or traps providing for the distinct antennas.
Moreover, in order to reduce the overall height from 105 inches to 5 ½ feet, rather than using traditional meanderlines, staggered shielded meanderlines are utilized to provide better choking or trap functions.
Specifically, it was found that one could not reduce the overall size of the multi-element antenna of Ser. No. 11/641,041 by simply scaling the meanderlines. Rather it was found that a stagger tuning arrangement for the meanderlines was needed that involved utilizing the lower meanderline in tact, but shortening the upper meanderline to approximately 70% of the size of the initially designed meanderline.
In particular, the effective length of the broadband whip antenna as a function of frequency is constructed to never exceed 1.2 wavelengths from 30 to 450 megahertz. This constraint guarantees there will always exist a main lobe on the horizon. In one embodiment, at 30 megahertz the effective antenna length is 62 inches, whereas from 240 to 450 megahertz the effective length is 22 inches.
The effective length condition is maintained by the use of folded or shielded meanderline structures inserted at strategic points along the whip. The folded meanderline structures approximate so called photonic band gap devices which are periodic resonant structures. Such devices have alternating band pass/band stop characteristics as a function of frequency. At about 220 MHz the meanderline transfer function enters the band stop region. A smooth transition in the 240 MHz region is accomplished by utilizing the above-mentioned stagger tuned meanderline structures.
More particularly, the meanderlines are two fold periodic. The only practical way to integrate the meanderlines on the antenna is to use a folded or shielded configuration. The periodic meanderlines must have identical folds to achieve the ideal transfer function. Because the folds are stacked one above the other, the inner fold sees the shielding effect of the outer fold. This shielding effect causes the inner fold to have more delay than the outer fold. For optimum performance the two folds should have the same delay. Thus, the inner fold must be physically shortened.
As to meanderline impedance, the impedance across the respective meanderlines is such that the upper meanderline choke response is shifted to higher frequencies because of the shorter length.
At lower frequencies both lines act as shorts and have zero impedance, while at high frequencies the impedance of both meanderlines is high to achieve the trap or choke function.
At about 230 megahertz the lower meanderline starts to act as a choke while the upper meanderline is at a low impedance, i.e. forms a short. Under this condition the antenna acts like a asymmetrical dipole from the lower meanderline to the top of the antenna.
At about 280 megahertz the upper meanderline starts to act as a choke such that both meanderlines act as chokes. Under this condition the antenna acts like a asymmetrical dipole between the two meanderlines.
If the meanderlines are the same length, the transition from a full antenna to an abbreviated antenna leads to Gibbs oscillations in the antenna gain.
The above staggered configuration solves this problem by being asymmetric as an intermediate state so that the transition from a full antenna to an abbreviated antenna is more gradual, mitigating the oscillation problem.
Additionally, at the low end of the 30 megahertz band a tuning sleeve is positioned between the base of the lowest element and the ground plane, with the tuning sleeve being provided with two parallel RLC circuits tuned to different bands. The purpose of the sleeve with the RLC circuits is to eliminate an unwanted null and provide low VSWR at the low end of the VHF band.
Moreover, it was found that a parasitic re-radiator can be formed at the top of the 225-450 MHz dipole to provide improved gain especially for the upper region of the UHF band.
What is therefore made possible by the above improvements to the originally designed vehicular multi-band antenna of U.S. patent application Ser. No. 11/641,045 is that the antenna itself is of a unitary construction in which the cylindrical elements of the dipoles are stacked one on top of the other without having to screw together antenna segments.
Secondly, the overall height of the antenna is reduced from 105 inches to 66 inches which is ⅔rds of the height of the originally designed antenna.
Fourthly, staggered shielded meanderlines permit antenna shortening without unwanted oscillations.
Thirdly, shielded meanderlines provide effective chokes or traps, where unshielded meanderlines failed.
Additionally, the utilization of a base tuning sleeve results in a better VSWR at the low end of 30 megahertz band.
In operation, from 30 to 190 megahertz a center-fed dipole is made up of 4 cylindrical elements, one on top of the other. In this case the shielded meanderline chokes act as shorts between the lower two and the upper two dipole elements to provide a long dipole.
As one precedes above 190 megahertz the two shielded meanderlines, rather than performing a shorting function, transition to open at these frequencies resulting in a shorter dipole antenna operating at 225-450 megahertz. Here the antenna only utilizes the center two elements of the 30-190 megahertz dipole. Over 450 megahertz the four tubular antenna elements previously described have virtually no effect on a top mounted dipole operating between 700 megahertz and 2 gigahertz.
Thus the antenna has three in-line dipoles, with the lower band dipole consisting of four elements, pairs of which being electrically shorted together to form the 30 MHz to 190 MHz dipole. Thereafter, the center elements of this dipole are the only ones that are active in the 225 to 450 megahertz band, with the other elements electrically open with respect to this dipole.
Finally, all of the above mentioned elements are electrically isolated from a top 700 megahertz to a 2 gigahertz dipole. Note all the antenna elements are in-line and coaxially aligned in a single vertically stacked package, with the tubular elements surrounded in one embodiment by wrapped fiber glass. It has also been found that an intermediate fiber glass wrapping layer with overlying copper tape may be conveniently utilized to tune the dipoles for each vehicle mounting scenario. Moreover, a ground plane like sleeve may be placed over the intermediate fiberglass layer below the L-band dipole to reflect the L-band beam upward.
In summary, a shortened multi-band antenna includes in-line dipoles, selected elements of which having shielded meanderline chokes to be able to switch from an extended dipole at the lower VHF frequencies to a shortened dipole for the UHF band. Additionally, the staggered asymmetric meanderline configuration permits overall size reduction, whereas antenna construction includes an intermediate fiberglass layer over which conductive foil is placed for tuning and for parasitic radiator purposes to improve the gain of the UHF dipole in the upper regions of the band at 450 megahertz. Additionally, at the low end of the 30 megahertz band a sleeve is positioned between the base of the lowest dipole element and ground, with the sleeve provided with two parallel RLC circuits tuned to different bands to improve VSWR at the low end of the VHF band and to eliminate unwanted nulls.
Regardless of the design of the Broadband Whip Antenna of patent application Ser. No. 12/436,375, improvements are still required to obtain continuous coverage up to 6 GHz while at the same time reducing nulls in the far field and improving VSWR across some troublesome bands.
It is noted that continuous operation up to 6 GHz is highly desirable because of the existence of WiFi bands from 5.5 to 5.75 (GHz as well as the 2.8 and 2.5 GHz WiFi hands. A simple whip antenna to cover all WiFi bands permits detection of WiFi transmissions as well as the ability to jam them from a vehicle, vessel or aircraft.